Werkzeug 2.0.0 breaks tests

[bblommers]

The S3 server test test_s3_server_post_without_content_length no longer works with werkzeug 2.0.0, as the calls return a 200 instead of a 411.

I'm submitting a PR to downgrade werkzeug for now, back to the known working version 1.0.1, until we have a solution.

[bblommers]

Werkzeug was downgraded as part of #3920

[bblommers]

The failing test was a red herring, caused by a mismatch in flask/werkzeug versions. Our CI cached flask to 1.x, but upgraded werkzeug to 2.x, (or vice versa.. I forgot) and they don't mix'n'match very well.

When upgrading both to 2.x, most of the tests work. The only thing failing is the exception handling in the managedblockchain service. This was always handled badly, but didn't break until the werkzeug upgrade.

I'm rewriting the exception handling now - TBC..